The invention relates to a multi-piece valve for reciprocating piston engines.
Among other things, U.S. Pat. No. 2,136,690 discloses a multi-piece solid-stem valve, in which the valve seat has a reinforced lining of a wear-resistant material. The reinforced lining comprises a prefabricated, centrally perforated disk composed of a resistant and thermally conductive composite material which is conically beveled at the outer edge, said reinforced lining disk extending to the edge of the valve head and forming the head-side sealing face. The composite material is formed from a matrix composed of tough, conductive metal, preferably containing copper, into which finely dispersed particles of a hard, resistant material such as tungsten are firmly embedded. These hard particles are intended not only to protect the matrix but also to prevent or at least delay destruction of the valve sealing faces. In the known valve the disk serving as reinforced lining is riveted on to the head-side end of the valve stem together with a backing disk of conventional valve material applied to the combustion chamber side, the stem material serving as rivet. Here therefore, the valve head itself is of multi-piece construction, comprising two disks. A relatively wide shoulder is forged on the valve stem in order to axially support the valve head, comprising reinforced lining disk and backing disk, and prevent it from tilting. With a pin serving as rivet shank, the head-side end of the valve stem projects through the center opening of the two disks, the outer end of this pin being deformed into a rivet head extending in a spot-facing of the backing disk opening. Although the valve head is connected to the valve stem by a positively interlocking connection acting in both directions of the axial force—tensile and compressive—a disadvantage of the known valve is that in order to guide the disk connection of the multi-piece valve head and prevent it from tilting, a relative wide radial shoulder has to be formed on to the valve stem, the radial width of which shoulder in the example of an embodiment represented by the prior art is equal to approximately one third of the stem diameter. The shoulder formed by upsetting assumes not only the function of an axial support designed to prevent tilting of the multi-layered valve head, but also, by virtue of the smooth transition from the stem cross-section to the shoulder circumference, the function of a flow baffle element on the upper side of the valve head around which the flow passes. Another disadvantage is that the high-frequency impact stresses can result in minute relative displacements between the connected parts in the direction of rotation, which can lead to wear at the contact faces and hence to a loosening of the connection.
The earlier patent application by the present applicant, DE 100 29 299 A1, not previously published, not only describes different design constructions for multi-piece valves of the type addressed here but also goes into the methods of manufacturing the types of valve presented. However, for production reasons all of the valves disclosed are provided with a hollow stem, which although advantageous in the case of the present invention is in no way an essential prerequisite. Advantages of the known valve are the low weight and/or the long service life of the valve, which ensue from the fact that lightweight materials, in particular ceramic and titanium aluminide, capable of withstanding high thermal and/or tribological stresses can be used for the valve head. One disadvantage of the known valve, however, is that possible differences in the coefficients of thermal expansion, which depending on the mating of materials can sometimes be considerable, can result at the operating temperature of the multi-piece valve in a relaxation of the pre-tensioning in the connection between valve stem and valve head. Under the stresses occurring in operation this could likewise lead to a relative shifting of the contact surfaces and consequently to contact wear and loosening of the connection.
For the sake of completeness, reference should also be made to EP 296 619 A1, which likewise shows a multi-piece valve, the structural components of which are composed of different materials. The tubular valve stem is preferably composed of chromium molybdenum steel. The valve head, which should preferably be composed of a titanium aluminide intermetallic material, can be produced by precision casting. The finished valve head is provided on the upper side with a blind hole to receive the head-side stem end. The valve stem can be fixed in the blind hole by shrink fitting, cold pressing, brazing or by a combination of these joining techniques. In one case shown in the drawing of this specification, the inner surface of the blind hole is moreover formed with an axial corrugation, the end wall of the stem tube being expanded under the effect of pressure and localized heating and at the same time being designed to positively interlock in the corrugations on the hole side. In the case of the multi-piece hollow-stem valve according to EP 296 619 A1, however, there is reason to doubt whether the connection between valve stem and valve head will be sufficiently durable under the considerable static and dynamic loads imposed by both the thermal and the mechanical stresses.